Update rand requirement from 0.6.5 to 0.7.0

Updates the requirements on [rand](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-random/rand/commits)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

Author: dependabot-preview[bot]

Cargo.toml

@@ -28,7 +28,7 @@ futures01 = { package = "futures", version = "0.1" }
 futures-preview = { version = "0.3.0-alpha.16", features = ["nightly", "async-await"] }
 juliex = "0.3.0-alpha.6"
 mio = "0.6.16"
-rand = "0.6.5"
+rand = "0.7.0"
 runtime-tokio = { path = "runtime-tokio", version = "0.3.0-alpha.4" }
 tokio = "0.1.19"